Crippled Fukushima nuclear power plant again dispensed some 100 tonnes of highly radioactive water on Thursday. Its operator Tokyo Electric Power Co (TEPCO) blamed human error for the latest overflow.

Tepco however assured the contaminated liquid did not reach ocean waters.

It said workers left open valves to a pipe that leads to the holding tank in the H6 area of the facility.

"There could have been some (human) error, but we have to check the situation," Masayuki Ono, Tepco spokesman, said.

Based on photos supplied by Tepco, Japan's Kyodo reported valve was "opened" around 11 a.m. on Wednesday and then "closed" at 12:30 a.m. Thursday. The tank leak was discovered at 11:25 p.m. on Wednesday. The contaminated water that overflowed from the tank spilled into a rain gutter.

The contaminated water contained 240 million becquerels of beta-ray emitting radioactive substances per liter of water, including radioactive strontium, according to tests conducted by Tepco. Actual radiation levels in the water were likewise found at 50 millisieverts of beta rays per hour. A becquerel is a unit used to measure radioactivity.

"We apologise for worrying the public with such a leak," Mr Ono said. "Water is unlikely to have reached the ocean as there is no drainage in that tank area."

"We are now in the process of recovering the leaked water and the earth it has contaminated," he added.

Last week's leak is the plant's second most serious after August 2013 when it leaked 300 tonnes of water.

According to Asahi Shimbun, as of Feb 18, there are a total of 520,000 tonnes of radioactive water stored in the 1,000 holding tanks at the plant. More than half or 340,000 tonnes is highly radioactive.

Ever since the unfortunate March 2011 incident, Tepco said it has employed a number of measures to prevent further leakage, including adding more water level gauges to each tank, increasing the number of workers who patrol and observe the tanks and pipes, as well as raising the height of the barriers surrounding the tanks, among others.

However, Tepco still finds it hard to control the situation because of the increasing volumes of water at the plant. "More and more groundwater is seeping into reactor buildings and other facilities, where it is then being contaminated by radioactive substances," Asahi Shimbun said.